As \(\mathbf {L}\)  is a symmetric semi-definite operator, its eigenvalues are nonnegative and real. Hence the Rayleigh quotient of an eigenvector \(\phi \)  of \(\mathbf {L}\)  associated to an eigenvalue \(\lambda \)  is \(\mathcal {R}_{\pi ,\mathbf {P}}(\phi ) = \lambda \) , which is the definition of frequency considered in [1]}.

